1. Technical Field
The present invention relates to a communication system and a communication device, particularly to a communication system and a communication device, in which relay attack is difficult to perform.
2. Related Art
Nowadays, an electronic key system is becoming popular. The electronic key system includes a function (hereinafter referred to as an automatic entry function) in which a door of a vehicle can be locked and unlocked without using a mechanical key or without operating a portable key by conducting wireless communication between an in-vehicle communication device provided in the vehicle and the portable key possessed by a user.
The automatic entry function is roughly divided into the following two kinds of methods. In the first method, the door of the vehicle is automatically locked and unlocked when a user who possesses the portable key performs a predetermined operation (such that the user touches the door or such that the user operates a button provided in the door). For example, when the user performs the predetermined operation to the vehicle, an authentication request signal is transmitted from the in-vehicle communication device to a predetermined area, the portable key that receives the authentication request signal transmits a response signal including the authentication information, and the door is locked or unlocked when the authentication is successfully performed.
In the second method, the door is automatically unlocked when the user who possesses the portable key comes close to the vehicle, and the door is automatically locked when the user moves away from the vehicle. For example, the authentication request signal is periodically transmitted from the in-vehicle communication device to the predetermined area, the portable key that receives the authentication request signal transmits the response signal including the authentication information, the door is locked or unlocked when the authentication is successfully performed, and the door is locked when the response signal cannot be received.
The vehicle including the automatic entry function has a risk of a theft or an intrusion by a technique called a relay attack. As used herein, the relay attack is a technique in which, although the user who possesses the portable key is outside a communication area of the in-vehicle communication device, a malicious third party enables the communication between the in-vehicle communication device and the portable key to be conducted using a repeater, and performs such a fraud that the door of the vehicle is unlocked.
Conventionally, for example, Japanese Unexamined Patent Publication No. 2011-52412 has proposed that a request signal is transmitted twice from the in-vehicle communication device to the portable key as a countermeasure against the relay attack. In Japanese Unexamined Patent Publication No. 2011-52412, the portable key makes a notification of a reply mode for a second-time request signal when replying to a first-time request signal, and the portable key replies in the notified reply mode in response to the second-time request signal. Therefore, although time required to complete the authentication is lengthened because the request signal is transmitted twice, the relay attack is difficult to perform because it is difficult for the repeater to recognize the reply mode and perform switching.
For example, Japanese Unexamined Patent Publication No. 2012-209924 has proposed that, in addition to the countermeasure against the relay attack, the portable key transmits a signal having an identical content a plurality of times while FSK-modulating the signal by different maximum frequency shifts, thereby preventing interference such as disturbance of the wireless communication between the portable key and the in-vehicle communication device.